Re: [PATCH v2] ext4: fix soft lockup while shrinking the extent status cache

[sashiko-bot@kernel.org]

> From: Jianping Liu <frankjpliu@tencent.com>
> 
> __es_shrink() walks sbi->s_es_list and reclaims extents from each
> inode. On a non-preemptible kernel that walk never gives up the CPU.
> 
> There is a soft lockup report on our company's machine. The machine
> with millions of active inodes, 256 CPUs, and heavy memory pressure,
> concurrent shrinkers run too long.
> 
> i_es_lock is dropped after es_reclaim_extents(), and s_es_lock is not
> taken again until the next inode. When nr_to_scan remains, call
> cond_resched_tasks_rcu_qs() before continuing the scan.
> 
> cond_resched() does not report the RCU quiescent state needed here.
> It is compiled out when CONFIG_PREEMPTION=y. Otherwise it schedules
> through preempt_schedule(), and RCU records that switch as a
> preemption. An RCU-tasks read-side section ends only at a voluntary
> context switch, cond_resched_tasks_rcu_qs(), idle, or return to
> userspace. The helper reports that voluntary quiescent state and still
> lets a non-preemptible kernel reschedule.
> 

Sashiko has reviewed this patch and found no issues.

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